1. Field of the Invention
This invention relates to liquid crystal light valve projectors. Specifically, this invention relates to prepolarizing prisms used with two color projection systems.
While the present invention will be described herein with reference to particular applications, it is to be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings of this invention will recognize additional applications within the scope thereof.
2. Description of the Prior Art
The development of the liquid crystal light valve has opened the door to substantial progress in the state of the art of high quality large screen projectors. The reflective mode liquid crystal light valve is a thin film, multilayer structure comprising a liquid crystal layer, a dielectric mirror, a light blocking layer, and a photoresponsive layer sandwiched between two transparent electrodes. A polarized projection beam is directed through the liquid crystal layer to the dielectric mirror. An input image of low intensity light, such as that generated by a cathode ray tube, is applied to the photoresponsive layer thereby switching the electric field across the electrodes from the photoresponsive layer onto the liquid crystal layer to activate the liquid crystal. Linearly polarized projection light passing through the liquid crystal layer and reflecting from the dielectric mirror is polarization-modulated in accordance with the information incident on the photoconductor. Therefore, if a complex distribution of light, for example a high resolution input image, is focused onto the photoconductor surface, the device converts the image into a replica which can be projected with magnification to produce a high brightness image on a viewing screen. U.S. Pat. No. 4,019,807 issued to D. D. Boswell et al on Apr. 26, 1977 discloses such a high performance reflective mode liquid crystal light valve.
A graphics display projector using a liquid crystal light valve of the above-type is described in an article entitled "Application of the Liquid Crystal Light Valve to a Large Screen Graphics Display", published in the 1979 Society for Information Display (SID), International Symposium, Digest of Technical Papers, May 1979, pp. 22-33. This display system, a type with which the present invention is particularly but not exclusively concerned, projects a large scale image having a yellow-white characters on a dark blue background. The system includes a cathode ray tube (CRT) which provides input imagery; projection optics which provide the bright collimated output beam and necessary light polarization; and the liquid crystal light valve which interfaces the input and output functions.
The system uses a powerful light source such as a xenon arc lamp to illuminate the liquid crystal light valve through collimating and polarizing optics. Light emitted from the xenon arc lamp is transmitted to a main polarizing prism where it is separated into `S` and `P` components. The `P` component passes through the prism while the `S` component is reflected toward the light valve. Information displayed by cathode ray tube is transferred by fiber optics to one side of the light valve which changes the polarization state from `S` to `P`. The light is then transmitted through the prism and imaged on a screen by a projection lens. In this capacity, the main prism functions as an analyzer, converting modulations of polarization to modulations of brightness or intensity.
The quality of the projected image is generally a function of brightness, resolution and contrast. Image quality can generally be improved by placing a prepolarizing prism in the optical path in front of the main polarizing prism. The prepolarizing prism is somewhat effective in overcoming the deficiencies in the main polarizing prism. That is, since the main polarizing prism is not 100% effective in transmitting light of one polarization and reflecting light of another, light of an undesirable polarization may reach the light valve and be modulated and reflected back through the main prism onto the projection lens. This often results in distortions of color and/or reductions in contrast and resolution.
Since the prepolarizing prism may, for reasons of cost, be of the same design as the main prism, it would typically have similar reflectance and transmittance characteristics. However, when the two prisms are used in combination, the additive effect is such as to greatly improve the quality of the projected image. The prepolarizing prism substantially removes light of one polarization from the light which illuminates the main prism. The main prism then acts on the beam to substantially remove the residual light of the undesirable polarization.
However, in some applications it is desirable to use a second liquid crystal light valve for enhanced information displaying capability and versatility. In this application, the use of the prepolarizing prism becomes problematic insofar as the second light valve would require light of the polarization that would otherwise be removed by the prepolarizing prism. As a result, the use of a second light valve has forced a compromise in the quality of the projected image. This problem is addressed by one of the coinventors in a copending application entitled, "Two Color Liquid Crystal Light Valve Image Projection System With Color Selective Prepolarizors". It discloses the use of two prepolarizing prisms. As such, there remains a need for a less costly, less complicated solution to the above-noted problems.